What first got me into SciFi was House Davion and the Armed Forces of the Federated Suns. Gotta love Wolf's Dragoons and the Kell Hounds, though. Plus, the old-school Com Guard were pretty pimp.
Of late, I rather enjoy the exploits of the Republic of Cinnabar Navy (Up the RCN!), the Ancient Sith Empire has a lot going for it (including Mandalorian allies), and the Kushan (Hiigarans) and the Vasari are both great fun to command in battle.
0 Lord, save Thy people and bless Thine Inheritance. To our Rulers grant victories over the barbarians, and by Thy Cross protect Thine own Estate.